For the past few years, Brenda Buck has been sampling the dust blowing across southern Nevada. Until recently, she focused on the risks of airborne elements such as arsenic. But then she started noticing an oddity in her samples, a sprinkling of tiny, hairlike mineral fibers.

She found them on herself as well. After a ride on horseback down a dirt road 20 miles south of Las Vegas, her clothes and boots were dappled with the fibrous material. Dr. Buck, a professor of geology at the University of Nevada, Las Vegas, turned to her colleagues to help identify it.

Their verdict: asbestos. And lots of it.

In a paper published late last year, titled “Naturally Occurring Asbestos: Potential for Human Exposure, Southern Nevada, USA,” Dr. Buck and her colleagues reported that the fibers were similar to those found at asbestos-contaminated Superfund sites and warned that they “could be transported by wind, water, cars or on clothing after outdoor recreational activities.” The research raises the possibility that many communities in the region, including Las Vegas, may face a previously unknown hazard.

Dr. Buck and her co-author Rodney V. Metcalf, a fellow U.N.L.V. geology professor, are now trying to quantify the range and the danger posed by natural asbestos-bearing mineral deposits spread across 53,000 acres, stretching from the southern shore of Lake Mead to the edges of the McCullough Range. “Nobody wants bad news — we’re all hoping the health risks will be very low,” Dr. Buck said in an interview. “But the fact is, we don’t know that yet.”

Similar concerns are arising in an unexpectedly wide swath of the United States: Naturally occurring asbestos deposits now have been mapped in locations across the country, from Staten Island to the foothills of the Sierras in California.

Elongated asbestos fibers are created by natural mineral formations. When they turn up in industrial products, it is because people have excavated them and refined them for use — a practice dating back more than 2,000 years. Ancient Greeks used asbestos to strengthen everything from napkins to lamp wicks.

Stories of asbestos-linked illnesses date back almost as long. But it was the post-World War II embrace of these fibers, in products ranging from insulating materials to ceiling tiles to roofing shingles, that provided undeniable evidence of health effects. By the 1960s, scientists had demonstrated that a chain of occupational illnesses, including a lung cancer called mesothelioma, could be directly linked to the presence of such mineral fibers.

The term asbestos technically refers to a group of six silicate-based fibrous minerals. But this definition may underestimate the extent of naturally occurring risks, scientists say. The mineral erionite, for instance, also forms needlelike structures, which have been linked to startlingly high levels of mesothelioma in Turkey and which have recently been discovered in the oil-and-gas boom regions of North Dakota. The discovery of airborne erionite fibers in North Dakota recently led the Centers for Disease Control and Prevention to describe it as “an emerging North American hazard.”

“Essentially, these fibers flow aerodynamically into the deep lung tissue and lodge there” said Geoffrey Plumlee, a geochemist with the United States Geological Survey in Denver. They remain embedded for years, like needles in a pincushion, spurring the onset of not only mesothelioma but also other lung cancers and diseases of the respiratory system.

By the 1970s such health effects were so well documented that the Environmental Protection Agency moved to limit asbestos use, and in 1989 the agency banned almost all industrial use of the minerals. But a recent cascade of research has renewed scientific worries.

For one thing, recent soil studies show that residential developments have spread into mineral-rich regions. California’s state capital, Sacramento, for example, spilled into neighboring El Dorado County, where, it turned out, whole neighborhoods were built across a swatch of asbestos deposits.

When these conditions began cropping up across the entire town in the late 1990s, investigators assumed that those sickened were all workers at a nearby mine. But the illnesses weren’t appearing only in mine workers. Family members were stricken, too, as were residents of the town who had nothing to do with the mining business.

Investigations by alarmed government agencies — including the E.P.A, the Geological Survey and the National Institute of Environmental Health Sciences — established that miners brought asbestos fibers back to town with them on clothes, vehicles and other possessions. But residents were also exposed to fibers blowing about the surrounding environment — and, to the dismay of researchers, people were being sickened by far smaller exposures than had been thought to cause harm.

“Libby really started the new focus on the issue,” said Bradley Van Gosen, a research geochemist with the Geological Survey in Denver. Dr. Van Gosen has been put in charge of a new U.S.G.S. mapping project, an ambitious effort to trace the minerals not only across Western mining states but also elsewhere, from the Upper Midwest to a rambling path up the Eastern Seaboard, starting in southern Appalachia and stretching into Maine.

Dr. Van Gosen said that most of the Eastern deposits were linked to an ancient crustal boundary, perhaps a billion years old, that underlies mountain ranges like the Appalachians. Wherever they are found, though, minerals in the asbestos family tend to form when magnesium, silica and water are transformed by superheated magma from the earth’s mantle.

In Western states, such filamented minerals tend to result from volcanic activity. In the Midwest, where fibers have recently turned up associated with mining interests in Minnesota and Wisconsin, geologists suspect they originated in ancient magnesium-rich seafloors. A recent study in Minnesota linked an increased risk of death among miners to time spent working in mines contaminated by such deposits.

“It has the potential to be a huge deal,” said Christopher P. Weis, toxicology adviser to the director of the National Institute for Environmental Health Sciences. “And we want to get the word out, because this is something that can be addressed if we tackle it upfront.”

Dr. Buck’s discovery of similar hazards in southern Nevada was the first time that naturally occurring asbestos had been reported in the region. At this point, she and her colleagues are simply trying to figure out the extent of the problem. A leading mesothelioma researcher, Dr. Michele Carbone of the University of Hawaii, is analyzing the fibers to help establish the magnitude of any health risk. Dr. Buck and Dr. Metcalf are expanding their sampling deeper into the Nevada desert, trying to build a better map of the hazardous regions.

“We live here. Our children are here,” Dr. Buck said. “We want very much to get this right.”

And they are approaching their discovery with personal caution. They now wear protective gear while sampling, and Dr. Buck has decided against taking her graduate students out for what appears to be risky fieldwork.

On a larger scale, researchers are investigating alternatives to creating large forbidden zones, such as wetting down roads or requiring that people in high-exposure areas wear protective masks and gear. But even small measures, like bathing after exposure and washing contaminated clothing separately, may help, Dr. Weis said.

“We can be smart and efficient about this, both at the government and at the personal level,” he said.